The inventive subject matter relates to power conversion apparatus and methods and, more particularly, to uninterruptible power supply (UPS) apparatus and methods.
UPS systems are commonly used in installations such as data centers, medical centers and industrial facilities. UPS systems may be used in such installations, for example, to provide backup power to maintain operation in event of failure of the primary utility supply.
UPS systems may have any of a number of different types of architectures. For example, AC UPS systems may have an on-line or double conversion architecture including a rectifier configured to be coupled to an AC power source and an inverter coupled to the rectifier by a DC bus and configured to provide AC power to a load. A battery or other DC source may be coupled to the DC source, which may provide backup power in the event of failure of the AC source. Standby AC UPS systems may include an inverter that is configured to be coupled to a load by a transfer switch that switches a load between the inverter and an AC source. AC UPS system may have other architectures, such as line interactive and delta conversion architectures.
Large data centers have proliferated with the advent of web services and cloud computing. Some newer large data centers occupy millions of square feet and house hundreds of thousands of servers. Typically powered by the local grid, these centers may include backup power supply systems including UPSs and diesel or gas powered backup engine-generator sets to support continued operation when utility power is lost.
In a double conversion UPS, rectifier and inverter controls may be significantly decoupled because of the presence of energy storage on the DC link between the rectifier and inverter. The rectifier can be used to address source compatibility issues (e.g., voltage droop, harmonics, power factor, distortion, etc.), while the inverter can be used to protect the load (e.g., voltage, distortion, load regulation, etc.).
When an engine-generator set is used in a data center or similar application, the load applied to the engine-generator set is often gradually increased to avoid transient overloading of the engine-generator set and tripping its protection mechanisms. A double conversion UPS can be used to gradually increase loading of an engine-generator set (to “walk in” the engine-generator set) by using the battery and inverter to provide power to the load while the rectifier gradually increases loading of the engine-generator set. Such generator walk-in techniques may be unavailable, however, in UPS systems that do not have a double conversion architecture.